Laser-machining is increasingly used for cutting, drilling, marking, and scribing a wide range of materials, including brittle materials such as glass and sapphire. Traditional mechanical machining produces unwanted defects, such as micro cracks that may propagate when the processed material is stressed, thereby degrading and weakening the processed material. Laser-machining of brittle materials using a focused beam of pulsed laser-radiation produces precise cuts and holes, having high-quality edges and walls, while minimizing the formation of unwanted defects. Industrial progress requires laser-machining of an increasing range of brittle materials, while demanding increased processing speed and precision.
Transparent brittle materials interact with focused beams of pulsed laser-radiation through non-linear absorption of the laser-radiation. The pulsed laser-radiation may comprise a train of individual pulses or rapid bursts of pulses. Each individual pulse or burst of pulses creates a defect in the transparent brittle material at the focus of the beam. An array of defects is created by translating the focused beam of pulsed laser-radiation along a cutting path in a workpiece, thereby weakening the material. A thin workpiece may then separate spontaneously, while a thick workpiece may be separated in an additional step that applies stress. One such method is to apply a laser-beam having a wavelength absorbed by the material along the cutting path, which causes mechanical stress through heating.
In recent years, chemically strengthened glass has been developed and is used extensively as a cover glass for display screens of consumer electronic devices. The chemical strengthening is achieved by an ion-exchange process. Silicate sheet glass is immersed in a salt solution containing potassium ions (K+). Larger potassium ions substitute for smaller sodium ions (Na+) located near surfaces of the glass, thereby causing compression within surface-layers in the glass. Between such surface-layers, the interior of the glass is in tension, compensating the surface compression. The high surface-layer compression makes chemically strengthened glass extremely hard (Mohs scale of about 6.5) and resistant to scratching and mechanical impacts. Sapphire (Mohs scale of 9) is an alternative hard cover glass material used in some devices.
Cover glass for consumer electronic devices typically has thickness between about 300 micrometers (μm) and 1.1 millimeters (mm). Well focused pulsed laser-radiation creates defects that typically extend for a few tens of micrometers in depth. Cutting through the full thickness of a workpiece requires the focused laser-radiation to be scanned along the cutting path many times while varying the depth-of-focus.
Commercial laser-machining processes have been developed using various means to generate long foci, thereby reducing the number of scans required along a cutting path and increasing the productivity of laser-cutting apparatus. A “Bessel beam” is generated from a beam having a Gaussian transverse mode using an axicon or an equivalent phase mask as the focusing element. An axicon is a conical prism that is rotationally symmetric about an optical axis. A phase mask is a type of diffractive optical element (DOE) and is generally rather expensive to fabricate. In practice, an additional telescope is often required to de-magnify a Bessel beam and eliminate severe intensity modulations caused by imperfect fabrication of the axicon or DOE. Defects created using a Bessel beam may have satellite structure, which can result in a poor quality cut edge.
An alternative way to generate long foci is to create a self-guiding “filament”. A focused beam of pulsed laser-radiation having high intensity in a material becomes further focused due to non-linear components of the refractive index. Positive feedback between non-linear focusing and intensity creates a plasma. A lower refractive index within the plasma causes defocusing. A balance between the focusing and defocusing sustains a plasma state within a filament. Propagation of the filament creates a void in the material along the optical axis of the focusing element. Filament laser-machining requires high pulse energies, approaching the practical limits of the current generation of ultra-short pulsed laser-sources, and fine control of all beam parameters. Relatively small variances in material properties (such as normal material inhomogeneities) and beam parameters (such as shot-to-shot noise and laser-to-laser beam quality) can cause a loss of control in a filament laser-cutting process.
There is need for an efficient laser-cutting method that will cut strengthened glass or sapphire in a single pass along the cutting path that uses lower pulse energies. Preferably, the method should be deterministic and insensitive to variances in material properties and beam parameters.